Devices for tightening bolts and/or nuts comprise a socket for tightening up the bolt and/or the nut, and a socket drive motor serving as a power source and coupled to the tightening socket by power transmission means.
Such tightening devices are already provided wherein the socket drive motor is deenergized by a controller when tightening torque not smaller than a predetermined value acts on the tightening socket to stop the rotation of the socket.
When the device is used for tightening a bolt and/or a nut, torsional deformation occurs in the shafting members included in the power transmission means to the bolt and/or nut. Although the torsion is removed to some extent on completion of tightening, residual torque due to the torsional deformation remains in the shafting members.
The residual torque produces frictional resistance (hereinafter referred to as "biting engagement") between the tightening socket and the bolt and/or nut, rendering the socket unremovable from the bolt and/or nut.
A great force of biting engagement occurs especially in the case of tightening devices wherein the casing of the device is provided with a projecting reaction arm and which are used for tightening with the reaction arm pressed against a contact member in the vicinity of the portion to be fastened, and also in the case of tightening devices having an inner socket engageable with a tip at the forward end of a bolt and an outer socket coaxial with the inner socket and engageable with the nut and which receives a tightening reaction by the tip at the bolt end and the inner socket.
To preclude such biting engagement, it is practice to reversely rotate the socket drive motor on completion of tightening to forcibly release the tightening socket from biting engagement with the bolt and/or nut, whereas the motor which is reversibly rotatable requires a more complex motor control system and makes the device more cumbersome to use than when the motor is rotatable in only one direction.
The present invention provides a device for tightening bolts and/or nuts having a tightening socket which is releasable from biting engagement with the bolt and/or nut by disengaging a clutch in a rotation transmission system on completion of tightening, and which is made readily removable from the bolt and/or nut.